I can think of a few good reasons for having coupons.  The
          most striking one is that it allows for process/product
          checks without destroying a perfectly good board.  A
          cross-section of a coupon designed to the same parameters of
          the circuit board and placed on the panel should closely
          reflect what a cross-section of live product would look
          like.  Another good application is with measuring controlled
          impedance.  Today's designs make it very difficult to do a
          good TDR test.  Ideally, a 4" to 6" trace with no treeing or
          branching makes for the best test.  These are not always
          evident on board designs, but a coupon can be designed that
          way.  I hope this helps.
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation